React Redux: how to prevent state from getting reset when props/state change?

In my React Native app, I have the following constructor:

constructor(props) {
  this.state = {flag: false}
}

and a <TouchableOpacity> that triggers a callback like this:

<TouchableOpacity
  onPress={() => {
    this.props.function1();
    this.setState({flag: true})
  }}
/>

function1() changes a prop that I'm getting from Redux:

function mapStateToProps (state) {
  return {
    var1: state.var1
  }
}

The problem is that whenever function1() fires, my constructor gets called again, so flag gets reset to false. I need it to stay as true. What can I do here?

EDIT: I have the following Modal that gets switched on and off with flag:

<Modal
  visible={this.state.flag}
/>